Osmolality Calculator

Calculate the osmolality of blood plasma based on sodium, glucose, and blood urea nitrogen (BUN) levels. This measurement helps assess fluid balance in the body.

How to Use This Calculator

Enter the patient's sodium level in mEq/L or mmol/L
Enter the glucose level in mg/dL
Enter the BUN (blood urea nitrogen) level in mg/dL
Optionally, enter the blood alcohol level in mg/dL if applicable
Click Calculate to see the osmolality result

Formula Used

Osmolality = 2 × [Na+] + [Glucose]/18 + [BUN]/2.8 + [Alcohol]/4.6

Where:

[Na+] = Sodium concentration in mEq/L or mmol/L
[Glucose] = Glucose concentration in mg/dL
[BUN] = Blood urea nitrogen concentration in mg/dL
[Alcohol] = Blood alcohol concentration in mg/dL (if applicable)

Example Calculation

Real-World Scenario:

A patient presents with symptoms of dehydration and has the following laboratory values:

Given:

Sodium (Na+) = 145 mEq/L
Glucose = 120 mg/dL
BUN = 28 mg/dL
Alcohol = 0 mg/dL (not applicable)

Calculation:

Osmolality = 2 × 145 + 120/18 + 28/2.8 + 0/4.6

Osmolality = 290 + 6.67 + 10 + 0

Result: 306.67 mOsm/kg (indicating mild hyperosmolality)

Why This Calculation Matters

Practical Applications

Diagnosing and monitoring dehydration
Evaluating electrolyte imbalances
Assessing kidney function
Detecting ingestion of toxins

Key Benefits

Helps determine appropriate fluid therapy
Can identify osmolar gaps indicating toxic ingestion
Assists in managing diabetic ketoacidosis
Guides treatment in critical care settings

Common Mistakes & Tips

Ensure you're using the correct units for each parameter. Sodium should be in mEq/L or mmol/L, while glucose and BUN should be in mg/dL. Using different units will result in incorrect calculations.

If a patient has consumed alcohol, it's important to include the blood alcohol level in the calculation. Alcohol contributes to osmolality and not including it can lead to an underestimation of the true osmolality and potentially missing an osmolar gap.

Frequently Asked Questions

The normal range for serum osmolality is typically 275-295 mOsm/kg. Values above this range indicate hyperosmolality, while values below indicate hyposmolality.

An osmolar gap is the difference between the measured osmolality and the calculated osmolality. A significant osmolar gap (typically >10 mOsm/kg) suggests the presence of unmeasured osmotically active substances in the blood, such as toxic alcohols (methanol, ethylene glycol), mannitol, or certain medications.

Osmolality is the concentration of solutes per kilogram of solvent (mOsm/kg), while osmolarity is the concentration per liter of solution (mOsm/L). In clinical practice, osmolality is preferred because it is not affected by temperature and volume changes. For most practical purposes, the values are similar, but osmolality is more accurate.

References & Disclaimer

Medical Disclaimer

This calculator is for informational purposes only and should not be used as a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition.

References

StatPearls: Serum Osmolality - Comprehensive overview of serum osmolality, its clinical significance, and calculation methods.
Medscape: Osmolality Calculation - Detailed information on osmolality calculations and clinical applications.
AACC: Clinical Chemistry of Osmolality - In-depth article on the clinical chemistry aspects of osmolality testing.

Accuracy Notice

This calculator provides an estimate of serum osmolality based on standard formulas. The actual measured osmolality may differ due to individual patient factors, laboratory variations, or the presence of unmeasured osmotically active substances. This calculator should not be used to make clinical decisions without proper medical supervision.

About the Author

Kumaravel Madhavan

Web developer and data researcher creating accurate, easy-to-use calculators across health, finance, education, and construction and more. Works with subject-matter experts to ensure formulas meet trusted standards like WHO, NIH, and ISO.

Connect with LinkedIn

Tags:

health medical-clinical-calculators osmolality medical body

Related Calculators